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Concrete Construction

Concrete Construction. 3 major costs of concrete Formwork costs including labor, equipment, and materials Cost of reinforcing steel and placement Concrete materials, equipment, and labor for placing, curing, and finishing F11-1 . Cast in Place. Traditional method

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Concrete Construction

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  1. Concrete Construction

  2. 3 major costs of concrete • Formwork costs including labor, equipment, and materials • Cost of reinforcing steel and placement • Concrete materials, equipment, and labor for placing, curing, and finishing • F11-1

  3. Cast in Place • Traditional method • Allows concrete to be formed to site conditions • Used on most construction sites for structural members • Concrete placed in forms to cure • Forms removed after enough strength in concrete to support self and additional construction loads

  4. Cast in Place • Walls and wall footings • Need to avoid voids and separation of materials due to large fall • Special care for areas under windows , pipe chases and other formwork installed inside wall forms • Can pump concrete from bottom of form to get rid of voids

  5. Cast in Place • Floors and roofs • One way slab – only supported in one direction F 11-3 • Slab and beam • Slab, beam & girder • Outside beam is called spandrel beam • Columns poured before slabs – shrinkage • If poured same time could stress floors and beams due to shrinkage

  6. Cast in Place • Floors and roofs • One way slab – only supported in one direction F 11-3 • Slab and beam • Slab, beam & girder • Outside beam is called spandrel beam • Two directional slabs (waffles) provide support in two perpendicular directions • supported by columns

  7. Cast in Place • 2 way • Basically joists in both directions • Flat slabs • Supported directly by columns • Uses column capitals to distribute load over larger area • More reinforcement needed to carry loads to columns • F 11-6

  8. Precast concrete • Cast into desired shape at plant and moved to site • Controlled environment -> better finish and quality • All prestressed and pretensioned members are precast • F 11-7, 11-8

  9. Precast concrete • Types • Joists and purlins F 11-7 • Roof and floor panels • Flat 1-4” thick, 15 – 32” wide, 4-10’ long • hollow core 4 – 12” thick, 4-8’ wide, 15 – 50’ long • tee, double tee 4-12’ wide span 12 – 100 feet • channel slabs 2 – 5 ‘ wide, 15-50 ‘ long • F11-8

  10. Precast concrete • Types • Walls • Usually curtain wall construction panels fit between structural components to form wall • But used in tilt up construction • Panels cast horizontally on existing slab and tilted upright • F 11-9

  11. Prestressed Concrete • Initial compression load applied to concrete • Places entire beam in compression • Makes beam stronger since more force is required to induce a tension component • Reduces deflection • F11-10

  12. Prestressed Concrete • 2 ways to do • Pretension – place prestressing material in tension in the form while concrete is poured • Once concrete hardened remove tension • Bond between steel and concrete keeps steel in place • Post tensioning – steel is placed inside a plastic tube during concrete pouring • Upon placement steel is tensioned and the steel is mechanically anchored to the concrete at each end • Load is removed and steel cut flush

  13. Architectural Concrete • Appearance effects • Shape, size, texture, and color • F 11-12 & 11-13

  14. Concrete Construction Practices • Transporting and Hauling • Need to avoid segregation of materials in concrete • Wheelbarrows, buggies, chutes, pumps, conveyors, buckets, trucks • Placing and consolidating • Need to make sure form surfaces and bracing is right before placing concrete

  15. Placing • Forms must be coated in oil to allow removal • If placing concrete on ground – must moisten ground • If pouring on top of cured concrete must place bonding agent between pours • Shotcrete (gunite) – concrete placed pneumatically • Consolidation – removing air voids • vibration

  16. Finishing & Curing • Finishing – bring surface of concrete to its final position and surface texture • Screeding – striking off excess concrete • Floating – smoothes and compacts concrete imbeds aggregates • Troweling – compacts surface F 11-16 • Brooming – surface texture

  17. Curing • To get good concrete must cure properly • Moisture and temperature are key • Moisture – wet straw or burlap, curing compounds • Vacuum dewatering – mat placed on concrete • Vacuum applied to mat – takes out excess water • Lower W/C ratio -> denser mix

  18. Hot weather Concreting • Curing accelerates when concrete temp above 50 – 60 F • How to lower temp • Use cold water in mix • Cool aggregates before mixing • Use Type IV (low heat cement) • Add a retarder • Decrease max time to discharge to 1 hour

  19. Cold weather Concreting • Concrete should not freeze for 1st 24 hours • Min temp for placement is 50F • Use type III (hi early strength) • Use accelerator • Heat water and aggregates before mixing • Use vented heaters to keep concrete warm

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